Schumacher, who suffered concussion and a broken back in a 200mph crash at Indianapolis four weeks ago, has targeted the Hungarian Grand Prix next month as his comeback race.

Watkins, a neurological expert, believes that could be too soon. He said: "Ralf must make sure he is fully recovered and rule out the chance of suffering side effects from the crash. Otherwise a secondary impact syndrome threatens and that could mean the vibrations lead to brain swelling and, in the worst-case scenario, death."

Schumacher's crash at the United States Grand Prix was his second serious shunt in a year following his accident in testing at Monza ahead of the 2003 Italian Grand Prix. Schumacher suffered concussion but sat out just one race, a decision that has since attracted criticism.

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"Ralf has had two very similar heavy impacts in the last year, in Monza and Indianapolis, and the effect if he returned before he was fully fit could be the same as you see in a boxer," Watkins said.

Jarno Trulli, who crashed his Renault during the British Grand Prix at Silverstone, has no such concerns as he prepares for this week's German Grand Prix at Hockenheim. Trulli went off spectacularly after his suspension broke but, unlike Schumacher, he was able to walk away.

"The car did an excellent job protecting me, and I felt fine immediately after the race. I just want to get back in the car, and put in a more positive weekend than we had in Silverstone," Trulli said. Renault have upgraded engine parts for Germany to counter the malfunction experienced by Trulli's team-mate, Fernando Alonso, at Silverstone. The failure of either Renault to finish allowed BAR to narrow the gap in the constructors' championship to 12 points.